The present invention relates to footwear and, more particularly, to footwear wherein one or more viscoelastic inserts are disposed in the sole.
A shoe can be viewed as being divided into two general parts--an upper and a sole. The upper is designed to enclose the foot in a snug but comfortable manner. The sole is fashioned to provide a durable wear surface, provide traction, protection and shock absorption for the foot and leg while typically retaining a degree of flexibility.
Shoe design is a highly-refined science which results in the combination of upper and sole that work together to support and protect the foot while fulfilling a variety of specific criteria. Athletic shoes, for example, vary in design depending upon the sport in which the shoes are worn. Tennis shoes, basketball shoes, baseball shoes, running shoes, racquetball shoes and weightlifting shoes are each designed to be used in very specific and diverse ways. The design is crafted to provide a particular combination of support, traction, protection, shock absorption and flexibility to enhance performance. In addition to being designed for a specific function (sports, dress, casual wear, etc.), shoes are designed to meet specific wearer characteristics. For example, shoes for heavier persons are designed differently than shoes for lighter persons, differently for wide feet than for narrow feet, differently for high arches than for low arches, etc.
The midsole structure (i.e., the structure of the sole interior) must be designed to absorb the force of impact. Shock absorption relates to the attenuation of impact forces that can be harmful to the foot and other body parts. Energy absorption, on the other hand, pertains to the general soaking up of both impact and useful propulsive forces. A midsole may utilize one or more materials or components to absorb these two factors to varying degrees. The "trick" to shoe design is to properly balance shock absorption and energy absorption.
A midsole with "high" energy absorbing characteristics has relatively "low" resiliency and generally does not return much of the energy placed into the midsole at the point of foot impact, resulting in a "flat" feel and less efficient foot motion. By contrast, a midsole with "low" energy absorption has relatively "high" resiliency and returns more of the energy imparted to the midsole at the point of impact.
The terms "energy absorption" and "shock absorption" have often been used without precise delineation. While both effects pertain to independent actions of the midsole's response to the force of impact, the term "impact response" describes the operation of a midsole to both effects and the term "viscoelastic" describes the accomplishment of these two effects by a midsole. A desirable midsole, therefore, is one in which impact response contains the appropriate balance of shock absorption and energy absorption.
Attempts have been made to design an appropriate impact response by using a sole or an insert for the sole which contain a fluid medium such as a liquid or a gas. While initial designs of this type were plagued with development problems such as in providing adequate support and comfort, later designs such as those disclosed in U.S. patents to Rudy, Nos. 4,183,156 and 4,219,945 overcame the unreliability problem through the use of a membrane and gas combination. A later patent to Parker, et al., No. 4,817,304 improved the Rudy structure.
U.S. Pat. No. 4,843,735, issued Jul. 4, 1989 to Nakanishi describes a system similar to that used in the subject invention, but for the exclusive purpose of "shock absorption." Alternating air filled cavities and gel filled cavities form a shock absorbing sole. Sealed gel filled cavities retain the gel, whereas unsealed air filled cavities (merely covered) provide room for the gel-filled cavities to laterally expand. Since the air filled cavities are not sealed, they do not generate the upward force provided by the subject invention. Moreover, to achieve such a high level of shock absorption, Nakanishi must employ a very stiff gel (penetration value of from 50 to 200). (Penetration values throughout the subject application are expressed in millimeters divided by 10 (mm.div.10)). In contrast, the subject application typically uses gels having a penetration value of greater than about 250. The penetration value is measured according to--JIS (Japan Industrial Standard)K2530-1976 (50 g load).
The use of sole inserts is known (see for example U.S. Pat. No. 4,680,875, issued Jul. 21, 1987 to Danieli). However, no sole insert has previously been designed to control energy absorption in the manner described by the subject invention.
While the prior art structures have greatly improved the sole's impact response, wearer comfort and impact response may be less than that desired because the sole fails to evenly distribute shock and pressure. Accordingly, the quest for an optimum sole structure which would meet the needs of virtually any shoe wearer in a variety of shoe applications is still being sought.